My internal drive on my Catalina MBP 16 filled to 99%, resulting in all kinds of problems, inexplicably since I only had about 250GB of stuff on a 1TB.
Using DaisyDisk I discovered that the ~/Library/Keychains folder had filled up with well over 500,000 login.keychain.sb-xxxxx-xxxxx files of about 1.4MB each, overwhelming everything. I suspected that as I'd turned on iCloud for Keychain perhaps that was running amok, and knowing if I turned it off it would delete local copies maybe that would work. It didn't. With no disk space, all sorts of things were going wrong or impossibly slow, and other symptoms were appearing, and of course not sure if those were what caused the keychain overflow or if there were other issues. In any case, I deleted the whole folder and restarted.
But then I could no longer log into iCloud; completely borked even after a password reset. I could get into Messages, Game Center, and the Store, pretty much anything but iCloud. And other nasty stuff was still going on (resetting after restart to some arbitrary saved state across the system, permission issues in the ~/Library folder, etc. Again, maybe caused by the disk filling.
Obviously I'm going to need to erase and re-install, which is going on now.
But what happened? aside from never synching keychains via iCloud (I don't think one can opt out locally, and I use 1Password anyway)? Can and should I delete all keychains in the cloud itself? I don't want to even accidentally turn on synching on another device and have it do the same thing on say my iPhone.